Crystal Replacement

Am I taking a crystal off a board to put a new one on as a repair. No!

On boards with miniature surface mount components, a crystal is often the largest item on the PCB. If you are short on space or height, then the problem is compounded even more. To compound the problem if you have multiple components requiring a crystal or clock in of different frequencies that are not related in frequency, then you need multiple crystals.

The portable power supply I am designing is in this category, where it needs 16 MHz for the processor, 6MHz for the USB to RS232, and 3.6864 MHz for the SPI to serial adapter.

My first prototype PCB has 3 crystals, but I am less than pleased at having 3 crystals on the board. I decided to do some research on alternatives to using a crystal. After doing some Googling, I finally arrived on the Silicon Laboratories website, where I found the Si50x series of MEMS Oscillator Products. There are four versions of the device, Si501 to Si503 are one time programmable devices with various options for the frequency of operation. The Si504 is ordered on a specific frequency, but has a serial input that allows the frequency to be changed from the fixed power up frequency at any time. The serial protocol is a bit difficult to generate, as it has to be done by bit banging at a fairly high frequency.

I chose to purchase a field programmer (Si50x-FPB1-CUST), and a quantity of blank chips which may be programmed to any of the versions, with any frequency between 32768 kilohertz and 100 megahertz. The chip has only 4 pins – Power, Ground, Clock output, and option pin that may be configured with different functions.

For a clock oscillator I decided to program up a test chip as an Si503, that is programmed with four possible frequencies that selected by placing either a weak or strong pull up/down resistor on the optional input. the four frequencies may be set to any frequency within the range of the chip.

The chips themselves are available in three footprint sizes, and are priced a bit over a dollar each here in Oz. I have only done some initial testing, but I am certain that I will be using these chips in most if not all my new designs.

Below is a screen shot of the programming interface showing the various options, and the settings I have chosen for a quad frequency chip